[PATCH] D34769: [X86] X86::CMOV to Branch heuristic based optimization

[Amjad Aboud via Phabricator via llvm-commits]

aaboud added a comment.

In https://reviews.llvm.org/D34769#794214, @hfinkel wrote:

> Interestingly, we just recently introduced a pass into the PowerPC backend that does essentially the same thing; lib/Target/PowerPC/PPCExpandISEL.cpp. Could you please take a look at that and see how it compares to this implementation? I'm obviously curious as to whether we could add some target callbacks and unify the underlying logic in a target-independent pass. FWIW, both implementations are around 500 lines.


Thanks for the information, I will take a look.

> One thing that's not clear to me: how are you determining whether or not the branch is predictable?

I did not determine if it is predictable, however, I did the transformation only when I can assure gain of more than 25% of the mis-prediction penalty.
On the other hand, I considered during development of this pass to try determine one case where branch is not predicted, which is tree-search like algorithms.
I did that roughly by looking for how the CMOV result value was used, and if it was used for another "pointer-like" load, I considered it a possible case for tree-search.
Notice, that this is not part of this patch, but we might consider adding it if found reasonable.

> One outstanding TODO item we have from the PPC functionality is to support __builtin_unpredictable (which turns in to metadata at the IR level). Have you looked at that?

I know about this built-in, but as you said, it is not lowered into the machine IR. I did not try to figure out how to preserve it though.


https://reviews.llvm.org/D34769